Because BTC is growing in value faster than the S&P 500. Thus, if you base it on BTC, it is falling. But, since the S&P 500 is growing in value faster than the USD, it looks like it is growing when based on USD.

It's all just relative. You can form a chart like this with any asset you want and it will look different in some way.

Excellent question. Typically when you chart the S&P you are seeing it priced in US Dollars, which lose value over time due to inflation.

But you don’t have to price it in dollars. You can compare it to any currency or asset that is traded on the open market. Just open your favorite charting tool and enter SPX/BTC OR SPX/GOLD etc.

Here are two more examples…

S&P 500 vs M2 total money supply:

And BTC vs M2 total money supply:

Is the stock market really helping me to beat inflation? It doesn’t look great.